1. Field of Invention
The invention relates to positive displacement pumps, and particularly to pump inlet valves.
2. Description of Related Art
Many industrial processes involve separation of liquid and vapor. One very common means of separation uses a knockout pot to capture entrained liquid while allowing vapor to flow out the top. The liquid pools at the bottom of the knockout pot, and eventually must be removed after it accumulates. The removal of this liquid can be difficult when the separation process takes place under vacuum. If the liquid does not have enough sub-cooling, there may be inadequate net positive suction head to avoid cavitation in a pump used to evacuate the knockout pot. Some styles of positive displacement pumps, such as piston pumps, may successfully overcome this.
Further difficulties arise, though, if very deep vacuum is employed. In these situations, the inlet check valve of a positive displacement pump may not open, because even if a full vacuum is developed inside the pump's fluid chamber, for example, in the cylinder of a piston pump, there may be little or no differential pressure across the check valve to open it. One common way to address this is by locating the pump a significant distance below the knockout pot, so that a column of fluid exists at the inlet check valve, providing enough pressure to open the check valve when sufficient vacuum exists inside the pump's fluid chamber. This method, though, may be inconvenient due to the physical space required, and it still may result in diminished flow rates, since the pump's fluid chamber may not fill completely before the check valve again closes. Also, in the event that any air enters the system by running the pump dry or during maintenance, it may be very difficult to purge this air from the system, as it may simply expand and contract as the pump operates.
U.S. Pat. No. 5,810,570 by Nguyen teaches a reciprocating piston pump with a spring-loaded magnetic inlet check valve that is unseated by a magnet located in the piston when the piston is close to its top dead center position. There remains significant room for improvement over the art taught in this patent, though, as the volumetric efficiency of such a pump is inherently limited, the inlet check valve only being open when the piston is in close proximity.
Other solutions involve secondary chambers into which effluent may drain from the main knockout pot. These chambers may be temporarily isolated from the knockout pot by closing a valve, relieved of vacuum with another valve, and emptied through yet another valve connecting the chamber to an evacuation pump. After the chamber is emptied, vacuum may be reestablished in the chamber by connection to a vacuum source, and the chamber may be rejoined to the knockdown pot by reopening the valve between them. While effective, this solution is complex.
In view of the forgoing, an improved pump design is needed, in which the inlet check valve can be simply opened at appropriate times during the pumping cycle without relying on a pressure differential, and while achieving a high volumetric efficiency.